Overlay networks may be used to establish layer-2 links between remote sites over a layer-3 network. Ethernet Virtual Interconnection (EVI) is one example of an overlay technology that can implement exchange of layer-2 VLAN traffic between two sites. Other examples include, but are not limited to, Virtual extensible Local Area Network (VXLAN), Ethernet Virtual Network (EVN) and Overlay Transport Network (OTV).
An overlay network includes a plurality of site networks (also referred to as sites) and a core network. Edge devices (ED) of the sites are connected to the core network. Packets from a site are mapped and forwarded to remote sites through overlay tunnels established between the core network to the remote sites. In EVI the overlay tunnel is a generic routing encapsulation (GRE) tunnel. A GRE tunnel can bear multiple EVI links.
in an overlay network, according to a split horizon function, packets received from a port of an overlay tunnel (which is simply referred to as an overlay tunnel port) are forwarded within the site, but will not be forwarded through another overlay tunnel port. This split horizon function is used to avoid forming forwarding loops among multiple EDs in the overlay network.